1986 Mercury 75HP 4 cyl w/triangular fuel pump

Howdy,

I have a 1986 Mercury 75HP 4 cylinder two cycle motor, SN 0A994929. It has a triangular fuel pump that is now listed as obsolete, but diaphragm kits and internal parts are still available.

When I purchased the motor, it had sat up for a long time without being ran. I decided to go ahead and rebuild the carburetors, replace rubber hoses, etc that may have dry rotted due to age and lack of use rather than risk clogging a carb with rubber debris. I didn't rebuild the fuel pump initially, but want to go back and do so after test running the motor on the lake. Here's the results of the test run:

Notes:

1 Fuel tank is new, bulb and all gas lines and filters from tank to fuel pump are new. Gas was fresh gas (bought near lake sydney lanier, no alcohol supposedly) with new Merc Quicksilver two cycle oil mixed using ratio recommendations for this motor. Only old part in fuel path was fuel pump. Carburetors were rebuilt with OEM parts and synced by a professional mechanic. Water pump was rebuilt as well with a complete rebuild kit.

2 Stator and trigger assembly are new, replaced due to wiring damage done by dirt dobber chewing up wires and shorting them, then original owner tried to start motor and fried wires so wires could not be repaired.) Rectifier was also replaced, due to corrosion caused by dirt dobber mud.

3 Compression test got 135 PSI on all cylinders and original spark plugs looked brand new when removed from motor for inspection and pre-oiling of cylinders with two cycle oil/carbon cleaner mix for storage while I was restoring the boat the motor came with. Transmission oil was replaced twice, to force out as much dirt as possible. Original tranny oil was dark, but not milky.

Lake test for motor:

Motor ran extremely strong on first day test, but was unable to check if motor had correct prop due to loose wire on tach. Fixed failed heat shrink insulation on tachometer wiring connector, which correct tach problem and reran lake test next day.

Motor ran strong cold for about 200-500 yards, but began to hesitate and bog down, as though fuel starved. Pumping bulb while operating motor did not correct problem and bulb was "firm" during test. Motor continued to have acceleration problems when accelerating from stop at WOT (holeshot) or when going from low rpm (2000) to WOT. Motor would bog out and would no longer run at WOT without stalling. Motor acted starved for fuel. The motor did not exhibit this behavior when cold, but did after a short run. Water pump flow showed excellent with extremely strong flow from "pee hole."

Note: I realize this may be a fuel issue or it may be a electronics/electrical issue even though the stator and trigger are new. So I'm looking for suggestions as to how to proceed as well as doing simple, inexpensive things that will eliminate other potential issues.

Motor had problems above 200 rpm after initially running okay, but would "chug" along at 2000 rpm if accelerated slowly and trailered boat okay.

I am now looking at getting parts for the fuel pump. The triangular fuel pump this motor has on it are now listed as obsolete, but internal parts are available from Mercury and aftermarket (sierra/mallory).

Here are questions I have:

1. It is possible to replace the triangular fuel pump with the more common and still available square fuel pump? If yes, what parts are needed to do so beside the fuel pump? Or am I better off keeping the original style pump and rebuilding it?

2. If no to question 1, what parts should I buy for the triangular fuel pump and are the OEM parts any worse/better than the after market parts?

3. The OEM part number for the triangular fuel pump diaphragm assembly is 55278A 5, but I can't find a list what it comes with. Does it come with a diaphragm, two gaskets, 1 o-ring and two seals like the aftermarket diaphragm sets I see listed?

4. If I'm working over the triangular fuel pump, what parts should I replace as "good practice" for doing a quality job? Just the diaphragm assembly and perhaps the check valve assembly?

My apologies for so many questions. I'm pretty mechanical, as my father was a mechanic, I'm mechanically inclined and I've rebuilt lots of stuff, but I'm not experienced working on boat motors.

I'm choosing to go ahead and rebuild the fuel pump, due to age and risk of dry rot, etc., but would like suggestions of how I should proceed should rebuilding the pump not correct the problem.

BTW, I was an electronics technician for many years and if need be, can build the necessary board to use a multi-meter as stator/coil test equipment and I am willing to take it to a dealer with specialized test equipment if need be.

Thank you for your help,

Dave

Re: 1986 Mercury 75HP 4 cyl w/triangular fuel pump

1. Yes but the top port cover for the squre pump s NLA so it would take a donor engine for parts
2. Buy the check valve kit to get the valves gaskets and diaphram
3. It's only the gaskets and diaphram
4. Item #2 :: iShopMarine.com ::
Here's a diagram for a DVA

Re: 1986 Mercury 75HP 4 cyl w/triangular fuel pump

Laddies,

Thank you for the information. What engines would the top port cover have to come from to fit my engine? Right now, I'm thinking I'd be better off to pick up parts to rebuild this pump and if it reaches a point I can no longer get parts for the triangular pump, then I can get a square pump and the top port cover from a scrap engine. I live near Lake Sydney Lanier and lots of dead engine grave yards around here.

Also, Is my thinking going in the right direction related to troubleshooting?

Re: 1986 Mercury 75HP 4 cyl w/triangular fuel pump

That port cover is only used on the 4 cyl. 75HP engines and there is no serial number split for it so you are just gone to to check each engine you find. Mercury discontinues none wear parts quite quickly so complete pumps are NLA also but the repair parts do not The pump kits are still available for the earlest Mark 40 and 55 engines from the mid 50s. In the mean time look around for the used parts for a conversion if thats what you want.

Just build a DVA it's only a few bucks for parts and then you can test the ign. system to be sure it's operating properly when ever you want.

Re: 1986 Mercury 75HP 4 cyl w/triangular fuel pump

Got in the check valve kit and an o-ring for the filter side. Pulled the fuel pump off the motor. My guess it hasn't been off the motor since 1986 and is probably the original fuel pump and original parts. The diaphragm appears to be a plastic coated mesh and was dry, hard/stiff and was shaped to conform to the shape of the interior of the fuel pump. Appears to be original and the new replacement is a soft rubber. The small gasket (Merc part #25848) on the screw holding down the filter cover is dry rotted as well, gonna try to get one locally and replace that. The filter is in good shape, but there's some trash the filter has caught, so it'll need cleaning during rebuild. The original large O-ring that goes under the filter cover on top of the filter is flattened out and appears triangular.

Right now, with the dry rotted condition of the rubber/plastic parts in the fuel pump and very stiff out of shape diaphragm, I'm guessing rebuilding this fuel pump should resolve a good bit of my problems.

I also opted to replace the spark plug coils, the spark plug wires and spark plugs with new as well. I've decided it's just easier than dealer with 20 plus year old parts failing or under-performing. I'm wishing I'd stuck to my original plan of rebuilding the fuel pump from the start, could have saved myself quite a bit of time on the lake.

Old motors that have set up a long time can be good, as long as they're not rusted, but their component rubber/plastic parts can definitely dry rot.